GEOTHERMAL TRAINING PROGRAMME Reports 2013 Orkustofnun, Grensasvegur 9, Number 27 IS-108 Reykjavik, Iceland
DIRECTIONAL WELL DESIGN, TRAJECTORY AND SURVEY CALCULATIONS, WITH A CASE STUDY IN FIALE, ASAL RIFT, DJIBOUTI
Farah Omar Farah Ministry of Energy and Natural Resources P.O. Box 10010 Djibouti DJIBOUTI [email protected]
ABSTRACT
Djibouti plans to drill 4 new geothermal wells for its future production in the Lava Lake or Fialé, within the Asal Rift segment. These wells are planned to be directionally drilled and the targets are based on six previous wells (Asal 1 - 6) drilled in the late 1980s in this rift segment. These 6 wells were drilled in an area which has high temperature potential, but problems of low permeability and high salinity were encountered. This paper presents directional well planning for these new wells and calculations for: (a) the trajectory and survey, (b) the well path, (c) the vertical depth. Two case studies are presented for Fialé and the resulting well path, using the build and hold type of directional wells. A simple BHA with two-stabilizers is proposed with an optimal weight on bit. Based on the casing plan, the kick-off point should be lowered to 430 m depth rather than 350 m as proposed by the pre- feasibility study, which is about 30 m below the casing shoe of the intermediate casing. Another option is drilling vertical wells, which would provide the same subsurface information as directional wells. However, a thorough cost analysis of drilling, survey tools, and equipment is needed to determine whether directional or vertical drilling is financially advantageous for the exploration wells. Directional drilling would not require new permitting or much geological and geophysical studies as such studies were done in the pre-feasibility phase in 2008. Drilling pads and targets for vertical drilling would, however, require further studies and a permit to drill inside of Fialé crater.
1. INTRODUCTION
Directionally drilled wells represent an efficient way to reach special targets that are difficult to reach using vertically drilled wells. A drawback of directional drilling is higher cost, but the advantage is that surface construction may be minimized while still reaching the intended targets. The main factor in the cost of a directional well is the horizontal distance to the target. The objective of the present study is to present the calculations that show the well path in a 3D space, and to develop the model that gives the minimum drilling length for these wells. The project of constructing and operating a geothermal power plant is divided into four phases from the exploration phase to production. The Asal project is currently in the exploration phase, which involves drilling four exploration and appraisal wells, followed by a
625 Farah Omar 626 Report 27 resource appraisal period, and finally by drilling seven to nine additional production wells. Information attained through the drilling of the exploration wells and during the appraisal of the second phase will be used for the conceptual design of the power plant. The true vertical depth (TVD) departure from the end of the built section, and the well path in a build and hold well profile is calculated.
Although the focus of this work is directional drilling, vertical drilling should not be excluded as a perfectly viable option. Therefore, this topic is also presented in a separate section and compared with directional drilling.
1.1 Background
The objective of steering a well trajectory in the right direction and hitting a geological target many kilometers downhole has forced the drilling industry to really focus on tools and methods to identify wellbore location and its path during drilling. In the early days of drilling exploration, it was common to set the drilling rig right above the target and drill a vertical well into it. Later, it became necessary to drill wells to reach targets that were deviated from the reference location at the surface. Throughout the years, many tools and methods have been developed for directional drilling. There are several companies offering tools to deflect and steer wellbores in the right direction and to measure wellbore inclination and azimuth.
The directional survey measurements are given in terms of inclination, azimuth and 3D coordinates, TVD, northing and easting at the depth of the survey station. For many applications, the accurate position and direction of the borehole should be determined at depths which may not coincide with the depth of survey stations. A mathematical tool for interpolating between survey stations is then required.
1.2 Objective
The objective of this paper is to show the calculation methods needed for directional well path design and to show the usage of trajectory and survey calculation methods by designing the well path of two wells in Asal Fialé. The emphasis is on the following:
Calculate the true vertical depth (TVD) and departure from the vertical, at the end of the build-up (EOB) section and the total depth (TD) to the bottom of the hole, in a build and hold well profile. Calculate directional coordinates. Describe formulas used to describe and calculate the well trajectory for different methods: Tangential; balanced tangential; average angle; radius of curvature; and minimum curvature. Outline the procedure for calculating survey results. Calculate the northing, easting, TVD, vertical section and dogleg severity of a survey station using the minimum curvature method. Determine the exact bottom hole location of the well. Monitor the actual well path while drilling to ensure the target is reached. Orient deflection tools (such as directional drilling assemblies) in the required direction when making corrections to the well path. Design the bottom hole assembly (BHA) including the buoyed weight (or hook load) in a vertical hole and the required BHA weight in air.
1.3 Scope
The minimum curvature method was chosen for trajectory calculations of the well. The scope of this work is based on: Report 27 627 Farah Omar
Literature review on directional drilling and survey calculation methods; and Trajectory and survey calculations methods.
1.4 Literature review
The following directional drilling methods are covered in the following books: Applied drilling engineering by Bourgoyne, Millhem, Chenevert, and Young (1991); Directional drilling and deviation control technology by the French Oil and Gas Industry Association (1990); and Directional drilling by Inglis (1987). Other references are indicated where used. It is pertinent to note that this literature is focused towards petroleum drilling practices. Other sources are:
1957: J.E. Edison presents the average angle method; 1968: G.J. Wilson presents the radius of curvature method; 1971: J.E. Walstrom presents the balanced tangential method; 1973: W.A. Zaremba presents the minimum curvature method; 1991: Xiushan Liu presents the constant curvature method; 1994: Wong et al., and Morita and Whitebay elaborate on the design of wells. 2004: S.J. Sawaryn and J.L. Thorogood present their SPE paper named A compendium of directional calculations using the minimum curvature method.
2. DIRECTIONAL WELL DESIGN
2.1 Directional drilling
Directional drilling is described as the deflection of a wellbore in order to reach a pre-determined objective below the surface of the earth”. Figure 1 shows the main parameters of a directional well.
2.1.1 Definitions and terminology
Directional drilling is the methodology for directing a wellbore along a predetermined trajectory to a target. Vertical wells are usually defined as wells with an inclination within 5°. Wells with an inclination greater than 60° are referred to as highly deviated wells. Wells with a section having an inclination greater than 85° for a significant distance are called horizontal wells. The following terminology is used: FIGURE 1: Measurement parameters of a
directional well (modified from - Azimuth: The angle (°) between the north direction Gabolde and Nguyen, 1991) and the plane containing the vertical line through the wellhead and the vertical line through the target. - Build-up rate: The angle from the kick-off point is steadily built up. This is the build-up phase. The build-up rate (°/30 m) is the rate at which the angle is built. - Drop-off point: The depth where the hole angle begins to drop off (i.e. tending to vertical). - Displacement: The horizontal distance between the vertical lines passing through the target and the wellhead. - Inclination: Angle (°) made by the tangential section of the hole with the vertical. - Kick-off point (KOP): The depth at which the well is first deviated from the vertical. - Measured depth (MD): Depth (length) of the well along the well path. Farah Omar 628 Report 27
- Tangent section: Section of a well where the well path is maintained at a certain inclination, with the intent of advancing in both TVD and vertical section. Short tangential sections are built for housing submersible pumps for example. - True-vertical depth (TVD): Vertical distance between kelly bushing (KB) and survey point. - Vertical Section (VS): Pre-defined azimuth angle along which the VS is calculated, usually the angle between north and a line uniting the wellhead and the total depth, measured on a plan view. - Well path: The trajectory of a directionally drilled well in three dimensions.
2.1.2 Application
The directional well is planned along a predetermined trajectory to hit a subsurface target. The target may be geometric and even adjusted in real time based on logging while drilling (LWD) measurements.
There are many reasons for drilling a non-vertical (deviated) well. Some typical applications of directionally controlled drilling are shown in Figure 2.
a) Multi-well platform drilling is widely employed in the North Sea. The development of these fields is only economically feasible if it is possible to drill a large number of wells (up to 40 or 60) from one location (platform) without moving it. The deviated wells are designed to intercept a reservoir over a wide area. Many oil fields (both onshore and offshore) would not be economically feasible without directional drilling.
FIGURE 2: Several applications of directional drilling as b) Fault drilling. When a well is common in the oil industry (Bourgoyne at al., 1991) drilled across a fault, the casing may be damaged by fault slippage. The potential for damaging the casing can be minimized by drilling parallel to a fault and then changing the direction of the well to cross the fault into the target. c) Inaccessible locations. Vertical access to a producing zone is often obstructed by some obstacle at the surface (e.g. river estuary, mountain range, city). In this case, the well may be directionally drilled into the target from a rig site some distance away from the point vertically above the required point of entry into the reservoir. d) Side-tracking and straightening. It is, in fact, quite difficult to control the angle of inclination of any well (vertical or deviated) and it may be necessary to ‘correct’ the course of the well for many reasons. For example, it may be necessary in the event of the drillpipe becoming stuck in the hole to simply drill around the stuck pipe (or fish), or plug back the well to drill to an alternate target.
Report 27 629 Farah Omar e) Salt dome drilling. Salt domes (called diapirs) often form hydrocarbon traps in what were overlying reservoir rocks. In this form of trap, the reservoir is located directly beneath the flank of the salt dome. To avoid potential drilling problems in the salt (e.g. severe washouts, moving salt, high pressure blocks of dolomite) a directional well can be used to drill alongside the diapir (not vertically down through it) and then at an angle below the salt to reach the reservoir. f) Relief wells. If a blow-out occurs and the rig is damaged, or destroyed, it may be possible to kill the “wild” well by drilling another directionally drilled well (relief well) to intercept or pass within a few feet of the bottom of the “wild” well. The “wild” well is killed by circulating high density fluid down the relief well, into and up the wild well.
2.1.3 Directional well types
There are several types of wellbore profiles. Below there is a description and an illustration of the most common profiles:
- Build and hold profile (Type 1) is the most common and simplest. The well is vertical until the KOP where it is kicked off and an angle is built. When the desired inclination is reached, the well path is kept tangent or straight until the target is reached. - Build, hold and drop profile (Type 2), also called shaped wells, is the same in the upper section as the build and hold well profile. The well is kept vertical until KOP and an inclination is built and the tangent section is drilled. After the tangent section, a drop-off section is drilled where the inclination is reduced and the well path is almost vertical as it hits the target. - Deep build/kick-off (Type 3) is a type of wellbore drilled when there is a hindrance, such as a salt dome, or when the well has to be side-tracked. The well is drilled vertically to a deep KOP and then inclination is built quickly to the target. Horizontal well profile and Horizontal Drain hole well profile are other types of wellbore trajectories. Theoretically, there are more than ten types of wellbore profiles.
These well trajectories are shown in Figure 3.
2.2 Planning the well profile
The first step in planning a directional well is to design the wellbore path, or trajectory, to intersect a given target. The initial design should consider the various types of paths that can be drilled economically. FIGURE 3: Most common types of 2.2.1 Parameters defining the well path wellbore profiles
There are three specific parameters which must be considered when planning one of the trajectories shown in Figure 3. These parameters combine to define the trajectory of the well:
- Kick-off point, is the long hole measured depth at which a change in inclination of the well is initiated and the well is oriented in a particular direction (in terms of north, south, east and west). In general, the most distant targets have the shallowest KOPs in order to reduce the inclination of the tangent section of the well (Figure 3). It is generally easier to kick off a well in shallow formations than in deep formations. The kick-off should also be initiated in formations which are stable and not likely to cause drilling problems, such as unconsolidated clays. - Build-up and drop off rate (in degrees of inclination) are the rates at which the well deviates from the vertical (usually measured in degrees per 30 m or 100 ft). The build-up rate is chosen on the basis of previous drilling experience in the location and the tools available, but rates between 1° and 3° per 30 m or 100 ft of hole drilled are most common in conventional wells. Since the build- Farah Omar 630 Report 27
up and drop off rates are constant, these sections of the well, by definition, form the arc of a circle. Build up rates in excess of 3°/30 m are likely to cause doglegs when drilling conventional deviated wells with conventional drilling equipment. The build-up rate is often termed the dogleg severity (DLS). - Tangent angle of the well (or drift angle) is the inclination (in degrees from the vertical) of the long straight section of the well after the build-up section of the well. This section of the well is termed the tangent section because it forms a tangent to the arc formed by the build-up section of the well. The tangent angle will generally be between 10° and 60° since it is difficult to control the trajectory of the well at angles below 10° and it is difficult to run wire line tools into wells at angles greater than 60°.
2.2.2 Target and geography
The trajectory of a deviated well must be carefully planned so that the most efficient trajectory is used to drill between the rig and the target location and ensure that the well is drilled for the lowest cost. When planning, and subsequently drilling the well, the position of all points along the well-path trajectory is considered in three dimensions (Figure 4). This means that the position of all points on the trajectory must be expressed with respect to a three dimensional reference system. The three dimensional system that is generally used to define the position of a particular point along the FIGURE 4: Well planning reference systems well path is:
- The vertical depth of the point below a particular reference point. - The horizontal distance traversed from the wellhead in a northerly direction. - The distance traversed from the wellhead in an easterly direction.
The depth of a particular point on the well path, referred to as true vertical depth (TVD) is expressed in metres (feet) vertically below a reference (datum) point. The northerly and easterly displacement of the point horizontally from the wellhead is reported as Northing/easting or longitude/latitude.
2.2.3 Defining the well path
Having fixed the target and the rig position, the next stage is to plan the geometrical profile of the well to reach the target. The most common well trajectory is the build and hold profile, which consists of 3 sections - vertical, build-up and tangent. The trajectory of the wellbore can be plotted when the following points have been defined:
- KOP kick-off point (selected by engineer); - TVD and horizontal displacement of the end of the build-up section; and - TVD and horizontal displacement of the target (defined by position of rig and target).
Since the driller will only be able to determine the long hole depth of the well, the following information will also be required:
- A long hole depth (AHD) of the KOP (same as TVD of KOP); - Build up rate for the build-up section (selected by engineer); - Direction in which the well is to be drilled after the KOP in degrees from north (defined by position of rig and target); - AHD at end of build (EOB) and the tangent section commences; and - AHD of the target.
Report 27 631 Farah Omar
These depths and distances can be defined by a simple geometrical analysis of the well trajectory.
2.3 Well path calculation
2.3.1 Build-and-hold
The following information is required:
a) Surface (slots) coordinates; b) Target coordinates; c) True vertical depth of target; d) True vertical depth to KOP; e) Build-up-rate.
The choice of slot depends on a number of factors including target location and the proximity of other wells. The target coordinates and depth are selected by the geologist. The choice of KOP and build-up rate has to be made by the directional engineer (Bourgoyne, at al., 1991):
Figures 5 and 6 show a build-and-hold wellbore trajectory intersecting a target at a true vertical depth (TVD) of TVD3 and at a horizontal departure of Dh (point D). The kickoff point is at a TVD of depth TVD1, where the rate of inclination angle build-up is q in degrees per unit length.
FIGURE 5: Geometry of build-and-hold FIGURE 6: Geometry of build-and-hold type well type well path for Dh ˃ R path for R ˃ Dh (same definitions as for Figure 5)
TVD AB: Distance from the surface location to the KOP; B-D: Distance from KOP to the bottom of hole; Dh : Deviation of the wellbore from the vertical (Horizontal displacement); TVD AG: True vertical depth; MD (A-D): Well measured depth; and q: Build up rate (°/30 m).
For the following formula, note that TVD3 =TVDAG, TVD2 =TVDAC, TVD1 =TVDAB. The radius of curvature, R, is thus:
180° 1 ∗ (1) π where q is the build-up rate in °/30 m. Farah Omar 632 Report 27
To find the maximum inclination angle, θ, consider in Figure 6 that:
90° θ 90° Ω τ or:
θ Ω τ (2)
The angle τ can be found by considering the triangle OPD, where (case R ˃ Dh):